1. Field of the Invention
The present invention relates to amplifier circuits and particularly to amplifier circuits for oscillators that are to have a high phase difference between input and output.
2. Description of the Related Art
The BAW (bulk acoustic wave technology) allows piezoelectric resonant elements that, for example, operate in the longitudinal wave mode and whose frequency (about 1-2 GHz) depends on the mass and elasticity coefficients of the resonator. These resonant elements are applied in RF (radio frequency) filters. A further field of application of so-called FBAR (film bulk acoustic wave resonator) are mass sensors (for example for biochemical or other applications). For this purpose, quartz crystal micro balances have primarily been used in the past. However, FBAR have a substantially higher measurement sensitivity compared to quartz crystal micro balances due to their higher resonant frequency. A further advantage of the FBAR is their integrability and thus more inexpensive manufacturing and the possibility of using them in sensor matrices.
The output signal of such an FBAR is the resonant frequency depending on the additional mass that is bound on the surface of the resonator and is to be measured. This requires the resonators to function not only in air (as is the case for RF filters), but also in water, for example.
Two methods are known for measuring the frequency:    1. Measuring the change of the S-parameters before and after the change of the surface mass. However, this approach has three major disadvantages: 1) the measurement is not accurate, 2) the expensive S-parameter measurement setup is not applicable for reading out in mass products, and 3) the measurement has to be done with the resonator in air. This requires the sensor to be dried before the measurement, which makes the whole procedure complex.    2. Use of a hybrid circuit oscillator. FIG. 3 shows a basic solution for an amplifier circuit having an amplifier 11 and a resonator 12, here specifically an FBAR (film bulk acoustic wave resonator). FBAR are used in biochemistry, for example as mass sensors for gases or liquids. Depending on the chemical structure of the gases or liquids, additional mass is bound on the FBAR. This results in a change of the FBAR's resonant frequency. The output voltage Vout generated by the amplifier 11 is attenuated/amplified and phase shifted by the resonator 12 and is returned to the input of the amplifier as input voltage Vin via a feedback loop.
The main problem for the use of FBAR in rough environments, for example water, is the poor quality, i.e. only a small phase shift and a high attenuation at resonant frequency.
FIG. 4 shows two curves of an FBAR. The upper curve represents the amplitude response (Vin/Vout). The lower curve represents the phase response (Vin/Vout). The frequency in Hertz is plotted along the x-axis of both curves. For the amplitude response, the amplification in dB is plotted along the y-axis. The phase shift in degrees is plotted along the y-axis of the phase response. The maximum phase shift between Vin and Vout is about −60°, for the resonant frequency of 1.89 GHz, the phase shift is even as low as −30°, while the attenuation is still 2 dB. To form an oscillating circuit with a total amplification of about 3 dB and a phase shift of 360°, the amplifier has to achieve an amplification of about 5 dB and a phase delay of −330°.
The specialist publication “Biochemical sensors based on bulk acoustic wave resonators” by R. Brederlow et al. shows an amplifier for an oscillator having an FBAR, wherein the amplifier comprises two transistors. The collector electrodes of both transistors are coupled to a supply voltage, wherein the FBAR is connected to the emitter electrode of the first transistor, wherein the base electrode of the second transistor is connected to the collector electrode of the first transistor via a decoupling capacitor and a voltage divider, and the output voltage is tapped at the emitter electrode of the second transistor. The base electrode of the first transistor is coupled to a conducting element.
The disadvantage of the described known circuit is the frequency dependence of the phase shift due to the invariable conducting element and the associated potential detuning of the oscillator circuit. If the amplification-phase relationship of the amplifier is not precisely tuned to the resonator, the amplifier may not oscillate or it oscillates at another frequency than the resonant frequency of the resonator, which is undesirable since the frequency then does no longer react to mass changes, i.e. no more measurements are possible. A further disadvantage is the major space requirement of a conducting element and the inflexibility with respect to the adjustment possibilities, because the operating point of both transistors is controlled by only one supply voltage.